[PATCH 7/7] writeback: timestamp based bdi dirty_exceeded state

[Wu Fengguang]

When there are only one (or several) dirtiers, dirty_exceeded is always
(or mostly) off. Converting to timestamp avoids this problem. It helps
to use smaller write_chunk for smoother throttling.

Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---

Before patch, the wait time in balance_dirty_pages() are ~200ms:

[ 1093.397700] write_bandwidth: comm=swapper pages=1536 time=204ms
[ 1093.594319] write_bandwidth: comm=swapper pages=1536 time=196ms
[ 1093.796642] write_bandwidth: comm=swapper pages=1536 time=200ms

After patch, ~25ms:

[   90.261339] write_bandwidth: comm=swapper pages=192 time=20ms
[   90.293168] write_bandwidth: comm=swapper pages=192 time=24ms
[   90.323853] write_bandwidth: comm=swapper pages=192 time=24ms
[   90.354510] write_bandwidth: comm=swapper pages=192 time=28ms
[   90.389890] write_bandwidth: comm=swapper pages=192 time=28ms
[   90.421787] write_bandwidth: comm=swapper pages=192 time=24ms

 include/linux/backing-dev.h |    2 +-
 mm/backing-dev.c            |    2 --
 mm/page-writeback.c         |   24 ++++++++++++------------
 3 files changed, 13 insertions(+), 15 deletions(-)

--- linux-next.orig/mm/page-writeback.c	2011-06-19 22:59:49.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-06-19 22:59:53.000000000 +0800
@@ -483,6 +483,15 @@ unsigned long bdi_dirty_limit(struct bac
 	return bdi_dirty;
 }
 
+/*
+ * last time exceeded (limit - limit/DIRTY_BRAKE)
+ */
+static bool dirty_exceeded_recently(struct backing_dev_info *bdi,
+				    unsigned long time_window)
+{
+	return jiffies - bdi->dirty_exceed_time <= time_window;
+}
+
 static void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
 				       unsigned long elapsed,
 				       unsigned long written)
@@ -621,7 +630,6 @@ static void balance_dirty_pages(struct a
 	unsigned long bdi_thresh;
 	unsigned long pages_written = 0;
 	unsigned long pause = 1;
-	bool dirty_exceeded = false;
 	struct backing_dev_info *bdi = mapping->backing_dev_info;
 	unsigned long start_time = jiffies;
 
@@ -669,14 +677,9 @@ static void balance_dirty_pages(struct a
 		 * bdi or process from holding back light ones; The latter is
 		 * the last resort safeguard.
 		 */
-		dirty_exceeded = (bdi_dirty > bdi_thresh) ||
-				  (nr_dirty > dirty_thresh);
-
-		if (!dirty_exceeded)
+		if (bdi_dirty <= bdi_thresh && nr_dirty <= dirty_thresh)
 			break;
-
-		if (!bdi->dirty_exceeded)
-			bdi->dirty_exceeded = 1;
+		bdi->dirty_exceed_time = jiffies;
 
 		bdi_update_bandwidth(bdi, dirty_thresh, nr_dirty, bdi_dirty,
 				     start_time);
@@ -719,9 +722,6 @@ static void balance_dirty_pages(struct a
 			pause = HZ / 10;
 	}
 
-	if (!dirty_exceeded && bdi->dirty_exceeded)
-		bdi->dirty_exceeded = 0;
-
 	if (writeback_in_progress(bdi))
 		return;
 
@@ -775,7 +775,7 @@ void balance_dirty_pages_ratelimited_nr(
 		return;
 
 	ratelimit = ratelimit_pages;
-	if (mapping->backing_dev_info->dirty_exceeded)
+	if (dirty_exceeded_recently(bdi, MAX_PAUSE))
 		ratelimit = 8;
 
 	/*
--- linux-next.orig/include/linux/backing-dev.h	2011-06-19 22:54:58.000000000 +0800
+++ linux-next/include/linux/backing-dev.h	2011-06-19 22:59:53.000000000 +0800
@@ -79,7 +79,7 @@ struct backing_dev_info {
 	unsigned long avg_write_bandwidth;
 
 	struct prop_local_percpu completions;
-	int dirty_exceeded;
+	unsigned long dirty_exceed_time;
 
 	unsigned int min_ratio;
 	unsigned int max_ratio, max_prop_frac;
--- linux-next.orig/mm/backing-dev.c	2011-06-19 22:59:49.000000000 +0800
+++ linux-next/mm/backing-dev.c	2011-06-19 22:59:53.000000000 +0800
@@ -670,8 +670,6 @@ int bdi_init(struct backing_dev_info *bd
 			goto err;
 	}
 
-	bdi->dirty_exceeded = 0;
-
 	bdi->bw_time_stamp = jiffies;
 	bdi->written_stamp = 0;
 


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